Portion of a back support for a chair



FIG. 1 is a front perspective view of the portion of a back support for a chair embodying the new design;

FIG. 2 is a rear perspective view thereof;

FIG. 3 is a right side view thereof;

FIG. 4 is a left side view thereof;

FIG. 5 is a front view thereof;

FIG. 6 is a rear view thereof;

FIG. 7 is a top view thereof; and,

FIG. 8 is a bottom view thereof.

FIGS. 1-8 show a seat support and portions of a seat back support in broken lines. The portions shown in broken lines are for environmental purposes only and form no part of the claimed design. 

CLAIM The ornamental design for a portion of a back support for a chair, as shown and described. 